When burned, all but a few fuels have solid residues, commonly called ash. The management of this ash is a major consideration in the design and operations of all combustion systems. A portion of the ash is carried with the flue gas to particulate collection equipment (fly ash). Another portion of the ash is continuously discharged through the bottom of the furnace (bottom ash) as a dry, friable residue (dry bottom furnace) or as a molten, flowing residue (cyclone furnace, or wet-bottom furnace). The remainder of the ash collects on the fireside surfaces as either a loosely adhering material that is easily removed by conventional mechanical methods (wall-blowers, sootblowers, lances, etc.) or a strongly adhering, tenacious deposit that resists the normal cleaning techniques. The present invention is directed at inhibiting the formation of these troublesome deposits and increasing the ease of their removal.
Two types of high temperature, fireside deposits are observed in combustion systems; slagging and fouling. Slagging is generally located in the radiant section or furnace and is typified by hard, dense, glassy deposits that may be molten on the outer surfaces. Fouling deposits are generally located in the non-radiant sections, such as the convection section of boilers, and are typically hard, sintered masses of fly ash.
Slagging results from the attachment of plastic or molten ash particles to the fireside surfaces. These surfaces may include refractory, water-wall tubes, steam tubes, superheater tubes, hangers, and other structural members. The attachment may be physical in nature, such as the penetration of the molten ash into the pores of the surface, or chemical in nature, with changes in the composition of the interface between the ash and the fireside surface. This attachment is enhanced by the presence of low melting point ash components, such as alkali metal and alkaline earth compounds, on the fireside surface. These low melting point compositions act as fluxes, increasing the degree of interaction between the surface and the ash. The plastic or molten ash particles solidify rapidly on contact with the fireside surface. A portion of the solidified ash may be crystalline, while the remainder is glassy in nature. The proportion of crystalline to glassy phase depends on the composition of the ash and the prevalent time/temperature conditions of deposition. The degree of adherence (adhesion strength) is dependent on the chemistry of the ash droplet and the relative difference between the surface temperature and the fusion temperature of the ash droplet. Additional ash deposits on this base. These particles may. be small glassy, fly ash spheres or the plastic or molten ash droplets. The interparticle strength increases as the particles sinter. As this insulating layer grows, the temperature of the outer portion increases, and the particles sinter more completely. The insulating layer continues to grow until the surface temperature of the deposit reaches the fusion temperature of the ash. At this point, the surface becomes plastic and then fluid.
The result of this process is the formation of a dense, adherent deposit. These deposits may cause several deleterious effects. Large sections of the deposit may break loose under their own weight or other stresses, blocking the bottom ash hopper throat or causing structural damage. The insulating nature of the slag reduces the heat transfer to the furnace walls, resulting in higher flue gas temperatures downstream of the slagged area. This will increase the potential for additional slagging and fouling, and possible loss of process control. These deposits may bridge across gas passages, restricting the flow of the flue gas, increasing erosion, and creating localized thermal stress. Slagging may cause spalling of refractory surfaces, or under-deposit corrosion may occur.
The standard methods of cleaning the fireside surfaces are mechanical or operational in nature. Devices such as wall-blowers or sootblowers are installed to blow the ash from the surfaces. Air, water, and steam are the standard media. These methods are frequently unsuccessful in removing the deposits. In addition, the frequency of use during slagging periods increases the rate of mechanical and thermal destruction of the surfaces. The level of excess air used for combustion is often increased to lower the temperatures in the furnace and change the oxidation state of the iron constituents in the ash. This should decrease the severity of the slagging, but reduces the efficiency of the operation. Load reductions, either temporary or permanent, reduce the flue gas temperatures and the ash burden. However, production rate reductions result. In many cases, the combustion system must be shut-down for off-line cleaning.
Fouling results from the condensation/desublimation of the more volatile ash constituents onto the fireside surfaces, with the subsequent impaction and adherence of dry, glassy fly ash spheres. These deposits are typically found in those sections of the combustion systems in which the flue gas temperatures are below the fusion temperature of the bulk of the deposit, such as the convection section or boiler bank. Fouling is initiated by the condensation/desublimation of the more volatile ash constituents, such as the alkali metal compounds, on to the relatively cold fireside surfaces. This forms a thin, tacky layer that enhances the capture of fly ash from the flue gas stream. These low melting point compounds may also condense onto the surfaces of fly ash particles, giving them tackiness and increasing the probability of their adhering to the fireside surfaces or other fly ash particles. As the fly ash collects on this base, the insulating nature of the deposit causes the temperature of the outer layer to increase, and the particles sinter. The extent of the sintering depends on several factors. Sintering is a temperature and time dependent process; higher temperatures increase the sintering rate, and the longer the deposit remains at the high temperatures the greater the densification. The presence of the low melting point substances enhance sintering through a fluxing mechanism. The reaction of flue gas sulfur dioxide and sulfur trioxide with the deposit, so called sulfation, increases the strength and tenacity of the deposit. If the flue gas temperatures become sufficiently high, the surface of the sintered deposit becomes plastic in nature.
The result of the fouling process is the formation of large, tenacious, sintered deposits. In the case of steam generator, these deposits inhibit heat transfer, making it difficult to maintain steam temperatures, and promoting additional deposition downstream. Fouling deposits can grow to the extent of blocking gas passages, restricting or diverting flue gas flow, increasing erosion, and creating localized thermal stresses. Under-deposit corrosion may also result.
The standard methods of cleaning fouled fireside surfaces included those described for removing slag. In addition to those, shot-gunning and rodding are frequently used methods. Although these are generally successful during the early stages, the extent of deposition usually proceeds to the point where the standard methods can not maintain satisfactory cleanliness. The shot-gunning and rodding techniques are undesirable because of the man-power intensive nature of those activities, the danger to the personnel, and the potential for puncturing the steam tubes. Ultimately, the combustion system will have to be shut down for cleaning.